It is known that internal combustion engines may be fueled by injecting a pressurized fuel stream to a high pressure air stream and metering the combination to an internal combustion engine. Fuel is injected to the air stream for a duration controlled by the width of a fuel pulse, and the fuel air combination is injected to the engine for a duration controlled by the width of an air pulse. The combination may be metered to a single air intake manifold for distribution to the cylinders of the engine or may be directly metered to each of the cylinders.
In such pneumatic fuel injection systems, the pressure of the air is regulated to a predetermined pressure above atmospheric pressure and the pressure of the fuel is regulated to a predetermined pressure above the air pressure. The difference between the air pressure and the fuel pressure may be used in a computation of the desired fuel pulse duration and the desired air pulse duration in accord with generally known engine air/fuel ratio control principles. For precise engine air/fuel ratio control, the pressure difference must remain substantially static or must be compensated, for example by sensing any significant perturbations in the pressure difference and adjusting the pulse duration as necessary.
Perturbations in the pressure difference have been attributed to several sources. Practical sources of the pressurized air, such as conventional air compressors provide an air stream having periodic pressure undulations around the desired compressor output air pressure. Likewise, conventional fuel pumps provide a fuel stream having periodic pressure undulations around the desired output fuel pressure. Other sources of pressure perturbations include changes in the load across the compressor or fuel pump supply voltage, such as from other electrical devices driven by the supply voltage. A temporary perturbation from a desired compressor or fuel pump drive rate may result from the load changes, at least temporarily affecting output pressure. Yet another perturbation source is the periodic opening and closing of fuel and air injectors in the system, which temporarily relieve pressure in the system.
To maintain a substantially constant air and fuel pressure despite these perturbations, air and fuel pressure regulators may be provided in the air and fuel paths, as described. However, practical regulators are incapable of rapidly responding to the described transient perturbations. Accordingly, further attempts have been made to minimize the pressure perturbations. These include providing a large damping orifice at the air compressor output, moving the air and fuel pressure regulators close to the injection points to improve regulator response, and providing diaphragm interfaces between the air and fuel paths wherein the diaphragms are deflected in response to pressure perturbations to minimize the difference in pressure. While these further attempts have reduced the pressure perturbations, a significant degradation in engine air/fuel ratio remains. Furthermore, these further attempts have added significant cost and complexity to the fuel delivery system.
Accordingly, what is needed is an approach to air and fuel pressure disturbance compensation that maintains an acceptable air/fuel ratio to the engine despite such disturbances, and does not materially increase system cost or complexity.